The present invention relates to roller bearings, and more particularly to a roller bearing suitably used, for example, for information instruments such as hard disc devices (HDDs), video tape recorders (VTRs), laser printers and the like.
Previously, roller bearings for supporting a spindle motor of hard disc devices and the like have employed a bearing ring and rolling element both of which are formed of steel.
In the above-described hard disc devices, to prevent the occurrence of writing errors and reading errors, the improvement of the acoustic characteristics such as vibrations, noises, etc., has been required.
Also, because these information instruments are frequently placed near users at a relatively calm place in an office, a home, etc., it is strongly desired to reduce the generation of noises, particularly, having the frequency in the audible range of a human.
Under such circumstances, to quiet a rolling bearing for supporting a spindle motor, it has been conventional practice to reduce the shape errors of each bearing component as much as possible, for example, by maximizing the roundness of inner and outer rings and by equalizing the sizes of all rolling elements.
In addition, when the constituting elements of a rolling bearing are formed of ball bearing steel, a heat treatment such as hardening, tempering, etc., is applied to the ball and ring elements which results in the residual austenite in the rolling element and ring being approximately 10%.
When such conventional roller bearings are subjected to inadequate lubrication, overheating results so as to cause defects such as fine impressions, cracks and peelings to form on the roller elements and the bearing rings, with such defects resulting in undesirable noise and vibration.
Also, as the result of investigations, it has been confirmed that one of the causes of noises generated by a rolling bearing is the fine scratches formed on, in particular, rolling elements. On the other hand, recently developed roller elements formed of hard ceramic material are resistant to being scratched. When the rolling element is formed with a ceramic material, the generation of noises by scratches is reduced because the ceramic material of the roller differs from the steel material of the bearing ring; also, sticking of the ball and ring component by coagulation because of inadequate or inferior lubricant can be reduced.
As described above, the formation of rolling elements with a ceramic material is effective for improving the acoustic characteristics of a ball bearing. However, the inventor""s further investigations revealed that use of ceramic rollers creates a further problem needing to be solved.
That is, when a rolling element is formed of ceramic, because the hardness and rigidity of the rolling element is increased and the rolling element is hard to deform, the contact area of the rolling element and the bearing ring becomes small and the stress applied to the orbital surface becomes large. Accordingly, when the bearing ring is formed of conventional steel wherein the residual austenite amount is about 10%, the contact portion with the rolling bearing element is liable to cause permanent ring deformation and resultant generation of unacceptable vibrations and noises. Also, when stress applied to the orbital surface becomes large, there is a problem that austenite is liable to cause a martensite transformation. Furthermore, in the case of using a ball bearing steel wherein the residual austenite amount is about 10%, there is a problem that the amount of the generated heat by the martensite transformation increases heat generation and undesirable excessive bearing temperature.
Accordingly, an object of the present invention is to provide a rolling bearing providing a reduction in undesirable characteristics such as vibrations, noises and the like.
Another object of the present invention is to provide a rolling bearing capable of stable and continuous excellent performance for a long period of time even under operating conditions of severe lubricant deficiency.
Still other objects, the features, and the merits of the present invention will become clear from the following descriptions.
That is, a first aspect of the present invention is a rolling bearing comprising a bearing ring formed with a steel material and a rolling element formed with a ceramic material, wherein the residual austenite amount of the steel material forming the bearing ring is not more than 8%.
Furthermore, a second aspect of the present invention in a preferred embodiment is a rolling bearing of the first aspect wherein the residual austenite amount of the steel material forming the bearing ring is from 0.05% to 6%.
In addition, if the residual austenite amount (YR) is less than 0.05%, there is an increased tendency for the formation of cracks so as to decrease the life expectancy of the bearing. On the other band, if the residual austenite amount exceeds 8%, the hardness is decreased so as to permit increased permanent deformation of the rolling bearing surface by either static loads or impact loads causing unacceptable scratches and impressions in the rolling bearing surface. The critical value of the residual austenite (YR) is consequently specified as described above.